This invention relates to supercritical fluid extraction.
In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In one type of apparatus for supercritical extraction, there is a specially constructed extraction vessel within a source of heat.
A prior art apparatus for supercritical extraction of this type is described by B. W. Wright, et. al., in ANAL. CHEM. 59, 38-44 (Jan. 1987) using a glass-lined extraction chamber within a bolted stainless steel extraction vessel heated in an oven. This type of extraction apparatus has the disadvantages of: (1) requiring time consuming steps to open the pressurized extraction vessel before use to insert the sample and again to open it after use to remove the spent sample; and (2) under some circumstances, requiring the handling of a hot extraction vessel.
In some supercritical fluid extraction apparatuses, extractant is collected under pressure in a cold trap. One such prior art device is described in "Supercritical Fluid Fractionation of Petroleum and Coal-Derived Mixtures", Anal. Chem. 1986, Vol. 58, pp. 2247-2255. This publication describes extracting under pressure to control the solvating power of the extractant and collecting under pressure. However, for very volatile compounds, this apparatus was reported to be unsuccessful and the extractant was bubbled through a collecting solvent while venting through Teflon tubing. It has a disadvantage in not solvating the extract in a solvent under a controlled adjustable pressure and temperature continuously during the extraction.
Another prior art collection technique is disclosed in Nam, Kapila, Yanders, and Puri, Chemosphere 20, No. 7-9, 873-880 (1990). In this technique, extract is collected under pressure and all the extract from a given extraction is collected in a vessel. Nam, et al. describe no provision for controlled vaporization of the extraction solvent to keep the fluid level constant.
The system described by Nam, et al. has several disadvantages, such as for example: (1) it does not provide for continual feed; and (2) it requires that the collection vessel be large enough to contain the total volume of extraction solvents used, plus any collection solvent, and the extracts extracted.
Usually, the volume of extraction solvent is many times the volume of analyte and the volume of collection solvent in which the analyte is to be concentrated. The volume for extraction solvent required may not be known until a test extraction is done. Fluid volumes are likely to be different between different extraction methods. For these reasons, it is desirable to evaporate the extraction solvent throughout the extraction using a collection vessel volume chosen to accommodate the analyte and collection solvent rather than the volume of extraction solvent.